西天山温泉布拉格达瓦一带托斯库尔他乌组沉积特征及构造环境分析

西天山温泉地区广泛发育泥盆—石炭纪火山—沉积地层,布拉格达瓦一带出露的托斯库尔他乌组发育一套由滨海间夹陆相—滨浅海相—浅海相过渡的碎屑岩夹火山碎屑岩的沉积建造,沉积物源主要为中酸性火山岩,碎屑颗粒沉积速度较快,属于近源沉积,蚀源区构造环境具活动性质,形成于未切割的活动岛弧。火山岩呈夹层、透镜体状分布于托斯库尔他乌组地层中,火山作用具周期性间歇喷发的特征,岩相表现为爆发—喷溢—沉积—爆发—沉积—爆发的周期性特点。火山岩属钙碱性岩石系列,火山岩浆对应的俯冲深度为上地幔。火山岩微量元素Nb,Sr,P,Ti,Ba为亏损型,Th,Zr元素富集,与岛弧环境密切相关;Nb/Ta为11.96~15.19,略低于原始地幔值,Zr/Hf为30.17~38.20,与原始地幔值相近。稀土总量∑REE为(186.57~208.58)×10^-6,轻重稀土比值为5.38~6.13;(Ce/Yb)_N为3.90~4.48,(La/Yb)_N为4.67~6.70,δEu为0.28~0.49,属于轻稀土富集型,显示为负铕异常。根据沉积物源区分析及主、微量元素特征,结合区域构造背景,认为晚泥盆世托斯库尔他乌组沉积建造所反映的构造环境为消减带俯冲碰撞的岛弧环境。

Sedimentary Characteristics and Tectonic Environment of Tuosikuertawu Formation in Bulagedawa Area in Xitianshan Hot Spring

Devonian-Carboniferous volcanic-sedimentary stratum are widely developed in Xitianshan hot spring.A set of clastic rocks intercalated with pyroclastic rocks transition from land facies to coast-shallow sea facies to shallow sea facies are developed in Tuosikuertawu formation exposed in Bulagedawa area.The sedimentary source is mainly intermediate acid volcanic rocks,the clastic grains are deposited rapidly,and belongs to near source deposits.The tectonic environment of the erosion source area is active and formed in the uncut active island arc.The volcanic rocks are distributed in Tuosikuertawu formation in the form of interlayers and lenses.The volcanism has the characteristics of periodic intermittent eruption,and the lithofacies is characterized by eruption-effusion-sedimentation-explosion-deposition-eruption.The volcanic rocks belong to calc-alkaline series,and the subduction depth of volcanic magma is upper mantle.The trace elements Nb,Sr,P,Ti,Ba of volcanic rocks are depleted,Th and Zr are enriched,which are closely related to the island arc environment;Nb/Ta value is 11.96~15.19,slightly lower than the original mantle value,and Zr/HF value is 30.17~38.20,which is close to the original mantle value.The total rare-earth elements is(186.57~208.58)×10^-6,the ratio of light to heavy rare earth is 5.38~6.13,(Ce/Yb)_N value is 3.90~4.48,(La/Yb)_N value is 4.67~6.70,andδEu value is 0.28~0.49.It belongs to the LREE enrichment type,and shows EU loss significantly.Based on the analysis of sediment source area,the characteristics of major and trace elements,and the regional tectonic setting.It is considered that the tectonic environment of late Devonian Toskultawu formation is the island arc environment of subduction and collision in the subtraction zone.